Combining Video Editor Scenes in One Scene
The gooseberry project uses a development branch of blender that has some modifications that haven't been applied to the main blender development yet. This is one change that is currently only in the gooseberry branch.

You can compare the code for the relevant panel in gooseberry branch and the master branch to spot the change. There are also other supporting code changes elsewhere to get that to work, see this commit.

There are gooseberry builds available to download from blender daily builds or you can look at building your own version of blender.